1. Field of the Invention
The present invention relates generally to the analysis of fuel failure in nuclear reactors and, more particularly, to a method of analyzing pellet-cladding interaction (PCI) in Condition II events.
2. Background Information
Commercial light water nuclear reactors (LWRs) generally include a plurality of cylindrical fuel elements which are grouped and secured together as separate fuel assemblies. The fuel assemblies are then arranged in an organized array to form the core of the nuclear reactor.
As shown in FIG. 1, each nuclear fuel element or rod 2 contains a stack of fuel pellets 4 (e.g., without limitation, uranium dioxide) enclosed within a zircaloy cladding 6 having a first end 8 and a second end 10. The first and second ends 8, 10 are capped by end plugs 12, 14, as shown. Typically, a hold-down spring 16 or other suitable restraining mechanism maintains the position of the fuel pellets 4 by biasing the pellets 4 toward the bottom or second end 10 of the fuel rod 2. A fission gas plenum 18 is disposed near the top or first end 8, and a relatively small or narrow gap 20 is present between the fuel pellets 4 and the cladding 6. When the fuel 4 is burned, it expands (i.e., swells) in both the axial and radial directions. Such expansion begins to reduce the gap 20 between the cladding 6 and the fuel 4. Eventually, if the fuel 4 is allowed to continue to expand, the gap 20 is totally eliminated (not shown). With different thermal expansion coefficients, increases in power can cause significant stress on the cladding 6. Once this stress exceeds a threshold, the cladding will rupture, an event commonly known as pellet clad interaction (PCI). PCI cladding failure is a breach of the first radioactivity boundary in the system, and results in the fuel pellets 4 and radioactive fission products being exposed to the reactor coolant. Such a condition is, therefore, extremely undesirable.
In an attempt to avoid cladding failure, the reactor protection system setpoints need to preclude operation that could result in PCI. The setpoints are validated using a detailed analysis of the normal operation (Condition I) and of events with moderate frequency (Condition II). More specifically, the analysis of Condition II events for fuel failure due to PCI is a regulatory requirement in some countries (e.g., without limitation, France). Condition II events include scenarios within the reactor core which are associated, for example, with an increase in nuclear reactivity and expressly include but are not limited to, boron dilution, rod withdrawal, and rod drop which is a condition in which one or more of the control rods unintentionally drop within the core.
Known methods of performing the PCI analysis are based upon a brute force approach that requires the review and analysis of many different operational histories and the initiation of different transients at multiple times within each history, followed by the evaluation of the impact of such transients on each and every fuel rod within the reactor core. In view of the fact that there can be up to about 50,000 or more fuel rods within the reactor core, it will be appreciated that such an approach is extremely labor intensive, time consuming, and costly. In some circumstances such rod-by-rod analysis can take up to about two years or more. Additionally, if the core or operational limits of the reactor change, the entire analysis must be redone.
There is a need, therefore, for a method of PCI analysis which is capable of accurately and efficiently evaluating and defining safe core operational guidelines in order to minimize the likelihood of PCI, without requiring a rod-by-rod brute force analysis.
There is, therefore, room for improvement in PCI analysis of nuclear reactors.